This invention concerns sewage treatment plants and processes, and in particular relates to an enhancement in the water side or liquid side of a wastewater treatment plant, resulting in reduced aeration requirements.
Currently, biodegradation of nitrogen-containing material in wastewater purification processes, e.g. activated sludge systems, is carried out in two or more stages. Typically the first stage is for denitrification under anoxic conditions (dissolved oxygen concentration=0 ppm) and the second stage is for nitrification under aerobic conditions (dissolved oxygen concentration>1 ppm). Usually aqueous medium containing microorganisms (referred to as “mixed liquor”) is recycled from the aerobic stage to the anoxic stage. Sometimes, a second anoxic stage is used after the nitrification stage in order to achieve low levels of total nitrogen (<8.0 ppm). Alternatively, multiple anoxic and aerobic basins are used in sequential manner sometimes in order to avoid the need of mixed liquor recycle. Typical volume ratio between the anoxic and the aerobic stage is 25:75. The A/O, A2/O, BARDENPHO, PHOSTRIP, PHOSTRIP II, DENITE, BIO-DENITRO, VIP and BIONUTURE are some of the processes which are based on this principle and are currently in use. The following U.S. patents have been issued in this area: U.S. Pat. Nos. 4,159,945, 4,051,039, 3,709,364, 3,964,998, 3,733,264, 3,864,246, 3,994,802, 4,056,465, 4,162,153, 4,257,897, 4,271,026, 4,488,967, 4,488,968, 4,522,722, 4,522,663, 4,556,491, 4,650,585, 4,867,883, 4,042,493, 4,141,822, 4,183,808, 4,956,094, 4,874,519, 3,977,965.
Such prior systems thus either used a series of multiple sequential anoxic and aerobic stages, e.g. six or seven such sequential stages, to avoid the need for recycle from aerobic to anoxic basins, or the systems included sequential anoxic and aerobic stages, fewer in number but with recycle of mixed liquor from aerobic to anoxic, which typically involved a much greater recycle flow rate than the quantity of new wastewater flowing into the system. In either case, energy requirements were much higher than those of the present invention, due to recycle pumping requirements and/or large aeration requirements. The invention described below eliminates the need for recycle and achieves efficient biodegradation of nitrogen-containing wastewater in a highly efficient manner.
In a relatively new wastewater purification process, known as the SYMBIO process, nitrification and denitrification are combined in a single stage by maintaining oxygen concentration below 1.0 ppm. The SYMBIO process is described in U.S. Pat. No. 5,906,746, and also referred to in U.S. Pat. No. 6,712,970.